1. Field of the Invention
The invention relates to underwater sound technology and, in particular, concerns an acoustic Doppler flow measurement system used to measure properties of rivers, estuaries, harbors, and man made channels.
2. Description of the Related Art
Acoustic Doppler flow measurement systems are commonly used to survey the total volume of flow in channels. The use of acoustic Doppler flow measurement systems mounted on small boats for channel discharge measurement has revolutionized hydrology. They offer faster, more accurate and safer methods of measuring channel discharge and surveying channel current structure than traditional techniques. Specifically, some of the advantages include: (1) measurements are made in a fraction of time associated with traditional methods, (2) many more measurements are made at once for improved accuracy, and (3) river traffic is not impeded by cross-river cableways used in traditional methods.
Traditional channel flow measurement systems typically employ single point current meters and depth sounders. The current meters are mechanically lowered down through the channel water column with a mechanical winching system mounted on a survey boat. The boat is typically attached to a cross channel cable, moving from one fixed location to another across the channel. The boat is held at a substantially fixed position while channel current velocities are measured at several water depths at each location by mechanically lowering single point current meters with a boat mounted mechanical winching system. Measurements with an acoustic depth sounder are also made at each location. Channel discharge is then estimated from these sample measurements by computing the product of the measured mean channel flow velocity (estimated from the current velocity measurements) and the channel cross sectional area (estimated form the water depth measurements).
The advantages of a the moving-boat acoustic Doppler flow measurement technique are derived form the unique capability to make substantially all the necessary measurements necessary for computation of channel discharge remotely from a moving boat, along the path of the boat as it travels across the channel. The acoustic Doppler flow measurement system remotely measures vertical profiles water flow velocity, earth referenced boat velocity, and water depth. These three parameters are all measured substantially continuously as the boat travels across the channel. Water velocity is determined throughout the water column by measuring the Doppler shifted echoes from small particles in the water column. Earth referenced boat velocity is determined by measuring the Doppler shifted echoes from the channel bottom. The boat's velocity vector can be determined from electromagnetic navigation systems such as GPS or from direct measurements of the Doppler shifted echoes from the stationary bottom of the river. In some cases, both GPS and bottom track Doppler measurements are combined via a Kalman filter to give an enhanced boat velocity estimate.
Boat position during the survey is computed from the earth-referenced boat velocity measurements. This position information is used to guide the boat operator across the river, and to position tag the water velocity profile and water depth data. Earth-referenced mean flow velocity is computed at incremental boat positions across the channel by subtracting the boat referenced horizontal water flow velocity profile measurements from the earth-referenced boat velocity measurements. River cross section is determined from the water depth and boat position measurements. Channel discharge is then computed from the product of the measured from the mean velocity and the channel cross sectional area.
At present, two types of boat mounted acoustic Doppler flow measurement channel survey systems are commonly used commercially. First is a low frequency acoustic Doppler flow measurement system that uses signals in the range 300-600 kHz to accurately measure river current profiles over depth ranges 0.5 to 30 m. The low frequency acoustic Doppler flow measurement systems typically have relatively large conventional multi-piston transducers. Second type is a high-frequency acoustic Doppler flow measurement system that uses signals in the range 1200-2400 kHz to accurately measure river current profiles over depth ranges of 0.1 to 10 m. The high frequency Acoustic Flow Measurement Systems typically have relatively smaller conventional multi-piston transducers.
One limitation of these existing systems is that in order to measure current profiles in a wide range of channel conditions (depth, flow velocity, sediment conditions, etc), for example 0.1 to 30 m depth range, two separate measurements need to be made using the two types (high & low frequency) of acoustic flow measurement systems. Another limitation of using two separate systems is the size and weight of the acoustic two separate transducers and the disturbance of the flow which they cause. The two multi-piston transducers are not easy to streamline to minimize instrument flow disturbance. An additional limitation is that speed of sound in water needs to be determined. This necessitates the measurement of the water's temperature and salinity, both of which can vary substantially in tidal rivers and estuaries.
Another limitation of a single frequency instrument is that trade-offs must be made between profiling ranges and spatial resolution. Deep rivers require use of low frequency systems in order for the sound to reach the full depth. The consequence is, however, that the vertical spatial resolution of the measurements is relatively coarse. Also, the minimum detection range may be substantial so that no data is collected near the surface. Conversely, high frequency systems provide improved spatial resolution and can make measurements near the surface but cannot penetrate to the deeper portions of the river. In addition, the high frequency signal may not be able to detect the bottom echo which is necessary to correct for the motion of the boat.
A further limitation is that the data from each can not easily be combined to allow for using the data from one system to improve the performance of the other. Hence, based on these limitations, there is a need for a small profile acoustic Doppler flow measurement system that permits channel measurements in wider depth ranges. There is also a need for an acoustic Doppler flow measurement system that does not need to depend on measurement of speed of sound in water to make accurate channel measurements.